This invention relates generally to commercial or residential integrated heat pump systems that provide domestic hot water heating, and comfort heating or cooling of a space, as required, and more particularly to a method and apparatus for delivering hot water heating from a variable speed heat pump system using outdoor air as the heat source while balancing space heating comfort and efficiency.
Heat pumps are often employed to provide heating or cooling, as needed, to a residential or commercial space or comfort zone, i.e., the interior of a home, office, hospital or the like. Heat pumps are also employed to heat water for domestic hot water and commercial use. A heat pump system for comfort zone cooling, comfort zone heating and domestic hot water heating is described in U.S. Pat. No. 4,766,734. Systems of this type can have several modes of operation, such as space cooling simultaneously with water heating, and space heating simultaneously with water heating. Additional modes, such as outdoor coil defrost, can also be employed. Under conditions of space cooling and water heating maximum system efficiency is realized since the heat removed from the space can be simultaneously rejected to the water system for water heating and the outdoor coil. Moreover, for space heating and water heating, supplemental resistive heating elements are also employed as auxiliary heating elements for use at set times when the heat pump alone can not produce sufficient heating for the comfort zone and or hot water. Although U.S. Pat. No. 4,776,734 provides for water heating during periods when space heating is also required, the auxiliary water heating elements are held inactive any time that the heat pump is operating so that the entire heating load of the hot water system is supplied by the heat pump. Typically, the stored water is heated to a temperature of between 120.degree. F. and 140.degree. F. Unfortunately, under conditions of simultaneous space heating and water heating the system efficiency and Coefficient of Performance is degraded since the heat sink temperature for the hot water tank is higher than that of the indoor coil.
Thus, there is a clear need for a method of operating an integrated heat pump and hot water system which delivers simultaneous space heating and water heating from a variable speed heat pump while balancing space comfort and water temperature with system efficiency.